Messier 51: The Whirlpool Galaxy

The Whirlpool Galaxy is the classic face-on spiral, with a smaller companion galaxy hanging at the end of one arm. It lies just below the end of the Big Dipper's handle and is one of the few galaxies where an amateur telescope can show spiral structure.

What it is

M51 is a large spiral galaxy about 31 million light-years away. Its 11-arcminute span works out to roughly 100,000 light-years, about the size of the Milky Way. It is interacting with its companion, NGC 5195 (also called M51B). The pull of the companion has shaped M51's strong, well-defined arms and compressed its gas, setting off waves of star formation that show up as pink and blue knots in photos. The core is mildly active, of Seyfert type. Three supernovae have been seen in M51 since 1994: 1994I, 2005cs and 2011dh. M51 is the leading member of a small group of galaxies that also includes M63.

When to see it

M51 is a spring object. It is highest around midnight in mid-April and well placed in the evening from April through June. From about 40°N it passes almost overhead, at about 83°, and it is circumpolar from latitudes north of about 43°. From the Southern Hemisphere it stays low on the northern horizon and is hard to see south of about 30°S.

How to find it

Start at Alkaid, the star at the end of the Big Dipper's handle. M51 lies about 3½° southwest of Alkaid, toward Cor Caroli. The 5th-magnitude star 24 Canum Venaticorum makes a good stepping stone: M51 is about 2° south-southwest of it. Under a dark sky the galaxy shows in a finder or in binoculars as a faint patch.

What you will see

In binoculars M51 is a small, faint glow; good binoculars under dark skies may show the two cores as a hazy double smudge. A 70–100 mm telescope shows two round glows, the larger M51 and the smaller NGC 5195, each with a bright center, almost touching. Under dark skies a 100 mm scope may hint at spiral structure with averted vision. In a 200 mm or larger telescope on a dark night the arms become clear, along with the bridge of light linking the arm to NGC 5195. Use low to medium power. M51 is sensitive to light pollution: from the suburbs you may see only the two cores, so save it for your darkest nights.

History

Charles Messier discovered M51 on October 13, 1773, while following a comet, and described it as a very faint nebula. His friend Pierre Méchain found the companion NGC 5195 on March 21, 1781, and Messier's later catalog noted that the object was double. In the spring of 1845 Lord Rosse, using his 72-inch telescope in Ireland, saw that M51 had a spiral shape, the first time spiral structure had been seen in any galaxy. His careful drawing of it became famous.